using fix smd to calculate PMF

Hi dear LAMMPS users,

Recently, I try to use fix SMD to compute PMF. In my simulation, I want to compute the free energy change when one molecule(x) desorpt from the surface. As shown in the diagram below, my simulation box is an equilibration state of many molecules adsorption on the surface. The surface stay static during the simulation through setforce zero and without displacement&velocity integration. x(as you can see in the diagram) upon the surface are adsorptive molecules. And the xyz are all periodic boundary conditions.

Diagram 1: